This invention relates to radioactive well logging and more particularly to well logging processes and systems for irradiating subterranean formations with bursts of fast neutrons and characterizing the fluid content of the formation on the basis of the thermalization period of the subsequently produced epithermal neutron population.
Various techniques may be employed in order to characterize subterranean formations with regard to their mineral content or lithologic characteristics such as porosity or to provide for stratigraphic correlation. The neutron source may be a steady-state source or a pulsed source. For example, neutron porosity logging may be carried out using a steady-state neutron source in order to bombard the formation with fast neutrons. The porosity of the formation then may be determined by measuring thermal neutrons employing two detectors at different spacings from the source or by measuring epithermal neutrons with a single detector.
In pulsed neutron logging procedures, the formations are irradiated with repetitive bursts of fast neutrons, normally neutrons exhibiting an energy greater than 1 Mev. When the fast neutrons enter the formation, they are moderated, or slowed down, by interaction with nuclei within the formation to form lower energy neutron populations. The fast neutrons are moderated to lower energy levels by the nuclear collision processes of elastic and inelastic scattering. In elastic scattering the neutron loses a portion of its energy in a collision that is perfectly elastic, i.e., all of the energy lost by the neutron is acquired as kinetic energy by the nucleus with which it collides. In inelastic scattering only some of the energy lost by the neutron is acquired as kinetic energy by the nucleus with which it collides. The remaining energy loss generally takes the form of gamma radiation emitted from the collision nucleus.
In the course of moderation, the neutrons reach the epithermal range and thence are further moderated until they reach the thermal neutron range. Thermal neutrons are neutrons which are in thermal equilibrium with their environment. The distribution in speed of thermal neutrons follows the so-called Maxwellian distribution law. The energy corresponding to the most probable speed for a temperature of 20.degree. C. is 0.025 electron volt. Epithermal neutrons are those neutrons which exhibit energies within the range from immediately above the thermal neutron region to about 100 electron volts. While the boundary between thermal and epithermal neutrons is, of necessity, somewhat arbitrary, it is normally placed in the range of 0.1-1 electron volt.
The populations of neutrons at the various energy levels decay with time following primary irradiation and thus offer means of characterizing the formation. For example, in the case of elastic scattering, which predominates for energies between a few ev and about 1 Mev, the number of collisions required for a neutron to moderate from one energy level to a second lower energy level varies more or less directly with the atomic weight of the nuclei available for collision. In subterranean formations, hydrogen nuclei present in hydrogenous materials such as oil, water, and gas tend to predominate in the slowing down process. Thus, the rate of decay of the epithermal neutron population gives a qualitative indication of the amount of hydrogenous material present which in turn may be indicative of the porosity of the formation. For example, U.S. Pat. No. 3,487,211 to Youmans discloses pulsed neutron logging techniques which involve the detection of thermal neutrons, epithermal neutrons, and fast neutrons. The fast neutron detection in Youmans is employed to monitor the output of the fast neutron source. The epithermal neutron detection is employed to obtain an indication of the decay of the epithermal neutron count in order to arrive at an indication of porosity. Epithermal neutron detection may be accomplished over successive time windows or over two overlapping time windows one of which completely encompasses the other. U.S. Pat. No. 3,800,150 to Givens discloses another pulsed neutron logging technique in which epithermal neutron decay or thermal neutron decay can be measured by employing time windows for detection which partially overlap each other. Thus in the case of the measurement of epithermal neutron decay, the measurement windows may exhibit durations on the order of 20 microseconds with the first time window starting during or immediately upon termination of the fast neutron burst and the second time window beginning perhaps 10 microseconds after the start of the first time window and extending 10 microseconds after termination of the first time window.
Low energy epithermal neutrons may also be detected in a manner to distinguish between oil and water within the formation. Thus, U.S. Pat. No. 3,497,692 to Mills discloses a pulsed neutron logging technique employing two neutron detectors both responsive to relatively low energy epithermal neutrons but within different energy ranges. For example, one detector may exhibit a substantial response within the range of 0.2-0.8 electron volt and the other within the range of about 0.1-0.6 electron volt.
The detector responses are gated over a plurality of time intervals to count rate meters. The outputs from the count rate meters are applied to a subtraction unit where the differences in count rates from the two detectors for each of the time intervals is obtained. The output from the subtraction unit is then recorded.
As noted in the Mills patent, when the energy of the neutron falls below about 1 electron volt, the hydrogen nuclei which are effective in moderating the neutrons to lower energy levels appear to be chemically bound to the other atoms in the molecule. In the case of a scattering reaction with a hydrogen atom in a relatively high molecular weight hydrocarbon molecule, the energy loss due to the scattering reaction is less than in the case of a collision between a neutron and a hydrogen atom in a relatively light water molecule. Therefore, a greater number of scattering collisions are required in oil than in water for the neutron to be slowed down to a given energy level. The variation in the output from the difference unit thus provides an indication of the character of the hydrogenous fluid within the formation.